Kids in urban centers are twice as likely to have shellfish allergies as rural children. (Photo: pinay06)

There's an old saying that country air will do you good. And a new study may be proving that right.

The work, which will be published in the July issue of Clinical Pediatrics, finds that kids in big cities are more than twice as likely to have peanut and shellfish allergies compared to rural communities.

This study is the first to map children's food allergies by geographical location in the United States, and points to environmental causes that could be setting off children's allergies.

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"We have found for the first time that higher population density corresponds with a greater likelihood of food allergies in children," said lead author Ruchi Gupta of the Northwestern University Feinberg School of Medicine.

"This shows that environment has an impact on developing food allergies. Similar trends have been seen for related conditions like asthma. The big question is - what in the environment is triggering them?"

Across the nation, one out of every 13 kids has a food allergy, said Gupta. And of the 38,000 children under age 18 that were part of the study, 40 percent who have a food allergy had experienced a severe, life-threatening reaction to food.

The kids, who come from a representative sample of U.S. households and live in urban centers, suburbs, small towns, and rural areas, showed marked differences in allergies based on where they reside.

In urban centers, 9.8 percent of children have food allergies, compared with only 6.2 percent of kids in rural areas who have them, though the severity of the allergies are similar regardless of where children live.

The states with the highest prevalence of food allergies are Nevada, Florida, Georgia, Alaska, New Jersey, Delaware, Maryland and the District of Columbia.

Though researchers aren't sure why certain allergies are more common in cities, they know that past research has shown higher rates of asthma, eczema, allergic rhinitis and conjunctivitis in urban areas versus rural ones, which leads them to the hypothesis that early exposure to bacteria associated with living in rural areas may protect against hereditary hypersensitivity to certain allergens. It could also be that pollutants in cities trigger allergies, they said.

Researchers also have wondered whether changes in the food supply, such as an increase in processed foods or a move away from locally grown foods, have played a role in the rise in food allergies in recent decades, Gupta said, according to MSNBC.

Gupta and her team plan to do further studies into the environmental causes that could be behind these food allergies.